Solid-State Transformation of an Additive Manufactured Inconel 625 Alloy at 700 ◦ C

: Inconel 625, a nickel-based superalloy, has drawn much attention in the emerging field of additive manufacturing (AM) because of its excellent weldability and resistance to hot cracking. The extreme processing condition of AM often introduces enormous residual stress (hundreds of MPa to GPa) in the as-fabricated parts, which requires stress-relief heat treatment to remove or reduce the internal stresses. Typical residual stress heat treatment for AM Inconel 625, conducted at 800 ◦ C or 870 ◦ C, introduces a substantial precipitation of the δ phase, a deleterious intermetallic phase. In this work, we used synchrotron-based in situ scattering and diffraction methods and ex situ electron microscopy to investigate the solid-state transformation of an AM Inconel 625 at 700 ◦ C. Our results show that while the δ phase still precipitates from the matrix at this temperature, its precipitation rate and size at a given time are both smaller when compared with their counterparts during typical heat treatment temperatures of 800 ◦ C and 870 ◦ C. A comparison with thermodynamic modeling predictions elucidates these experimental findings. Our work provides the rigorous microstructural kinetics data required to explore the feasibility of a promising lower-temperature stress-relief heat treatment for AM Inconel 625. The combined methodology is readily extendable to investigate the solid-state transformation of other AM alloys. expansion of the δ phase unit cell, which is consistent with a slow diffusion of Nb and Mo from the matrix phase to the δ phase. The in situ SAXS results reveal that the morphological evolution of the δ phase precipitates behaves differently at 700 ◦ C compared to 800 ◦ C and 870 ◦ C. The major dimension of the platelet δ phase precipitates reached a stable value of 154 ± 7 nm after 10.5 h at 700 ◦ C, which is in contrast to a continuously increasing major dimension that reached 961 ± 94 nm after 10 h at 870 ◦ C. In the context of a residual stress heat treatment, a stress relief heat treatment at 700 ◦ C for as long as 10 h results in δ phase precipitates (major dimension ≈ 150 nm) significantly smaller than those developed during typical residual stress heat treatment of AM 625 (major dimension ≈ 500 nm after one hour at 870 ◦ C or two hours at 800 ◦ C). We also compared the experimental findings with a TC-PRISMA-based precipitation simulation. The simulation captured the general trend in precipitation kinetics with good agreement between the observed and simulated precipitate size. The simulation overestimates the volume fraction of the precipitates, which is possibly due to factors such as the assumed spherical geometry of the precipitates, the effects of the dislocation density, and any temperature dependence of the interfacial energy. In general, this work unequivocally established significantly slower precipitation kinetics of the δ phase for AM IN625 at 700 ◦ C than at 800 ◦ C or 870 ◦ C, which are temperatures commonly used for residual stress relief, and this work also provides the rigorous microstructural kinetics data required to explore the feasibility of a lower-temperature stress-relief heat treatment for AM IN625.